Powder Metallurgy (P/M) is the technology of transforming metal powders into semi-finished or finished products by mechanical and thermal operations. Advantages of using P/M techniques include the ability to fabricate specialty alloys with unique compositions, microstructures and properties; to make parts of complex shape to close tolerances without secondary processing; and to produce alloys, such as the refractory and reactive metals, which can only be fabricated in the solid state as powders. Standard P/M techniques involve the pressing of metal powders in a die, the removal of the green part from the die, and the sintering of the part in a furnace under a controlled atmosphere. The starting powder may be a blend of pure elemental powders, a blend of master alloy powders, fully alloyed powders or any combination thereof. Non-metallic particulate materials may be added to make composites. The sintering process causes metallic bonds to form between the powder particles. This provides most of the strength. Bonding and/or densification may be aided by the development of liquid phases during sintering. These may or may not persist to the completion of sintering. These liquid phases may form by melting of elements or compounds, by the incipient melting of pre-existing eutectic compounds, or by the melting of eutectics which form by diffusional processes during sintering. The alloy may be used in the as sintered state or may be further processed. Secondary processes include coining, sizing, re-pressing, machining, extrusion and forging. They may also be surface treated and/or impregnated with lubricating liquids. Many metals are fabricated this way, including iron and steel, copper and its alloys, nickel, tungsten, titanium and aluminium.
The difficulty in sintering metal powders is a consequence of the surface oxide film which is present on all metals. This oxide film is a barrier to sintering because it inhibits inter particle welding and the formation of effective inter particle bonds. The problem is particularly severe in aluminium because of the inherent thermodynamic stability of the oxide (Al.sub.2 O.sub.3). Current P/M processed aluminium alloys are used principally in business machines where high mechanical strength is not required but where low inertia and corrosion resistance are important properties. There is, however, a demand for high strength, pressed and sintered aluminium alloys.
A general maxim in materials engineering is that alloys are tailored to the manufacturing process as much as to the application because different processes require different properties. Thus cast steels are different to both rolled steels and P/M steels; directionally solidified single crystal nickel superalloy turbine blades have a different composition to conventionally cast blades and aluminium extrusion alloys are different to forging alloys which in turn are different to casting alloys and rapidly solidified alloys. However, this principle has not yet been applied to pressed and sintered aluminium alloys. Current commercial alloys are predominantly based on the wrought alloys 6061 and 2014, which are Al-Mg-Si and Al-Cu-Si-Mg alloys, respectively. They have not been optimised for the P/M process.
U.S. Pat. No. 5,304,343 describes a method of producing a sintered aluminium alloy having improved mechanical properties. However, the alloy according to this patent is made using an expensive master alloy route and is based on 2,000 and 6,000 series alloys.
There is thus a need for an aluminium alloy powder blend, and sintered aluminium alloys produced therefrom, which provide higher tensile strength alloys for use in a broader range of applications than has hitherto been possible.